Engine No. 44, a Baldwin 2-8-0 steam locomotive engine built in 1921, has two wheels on the leading truck, eight driving wheels, and no trailing truck. The engine works on the Georgetown Loop Railroad and formerly ran in Central America. Diesel-electric locomotives began to replace steam locomotives in the 1930s and 1940s.
Steam locomotives can be classified in a number of ways. The most generally used classification, however, is based on the number and arrangement of wheels with which the engine is equipped. This classification gives the number of wheels on the leading bogie, the number of driving wheels, and the number of wheels on the trailing bogie. Thus, a 2-4-0 locomotive is one with a two-wheel leading bogie, four driving wheels, and no trailing bogie.
Many locomotives are also given special type names. Switch engines, used in railroad yards, are usually of the 0-6-0 or 0-8-0 type. Passenger locomotives include the American, 4-4-0; the Northern, 4-8-4; the Atlantic, 4-4-2; the Pacific, 4-6-2; and the Mountain 4-8-2. Freight locomotives include the Mogul, 2-6-0; the Consolidation, 2-8-0; the Decapod, 2-10-0; the Mikado, 2-8-2; and the Santa Fe, 2-10-2. A special type of locomotive used for heavy freight hauling is the articulated, or Mallet, locomotive, which is made up of two or more separate engines joined together, each with its own set of driving wheels. Among the various Mallet types are 0-6-6-0, 0-8-8-0, 2-6-6-2, 2-8-8-2, 2-10-10-2, and 2-8-8-8-2.
Until about 1940, steam engines provided the driving power of most locomotives used on U.S. railroads. Subsequently, the steam locomotive became largely obsolete. By the late 1980s, only a few of them, such as those on the narrow-gage tourist routes of Colorado, were operating in the U.S.